This invention relates to tip treatment bars of a rotor casing for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor through a series of slots defined between solid tip treatment bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
Known tip treatment bars are solid and relatively robust and, in general, have poor damping characteristics. Consequently, they are susceptible to high cycle fatigue failure. As the ends of the blades pass the tip treatment bars, the bars are aerodynamically excited. Vibration is induced in the bars in operation of the engine at a frequency determined by the passage of the blades. The vibrating bars deflect in a generally circumferential direction and consequently fatigue failure tends to be initiated by cracking at the slot ends.
It is an object of the present invention to increase the resistance of tip treatment bars to high cycle fatigue stress.
It is a further object of the present invention to reduce the amplitude of lateral vibrations of tip treatment bars.
According to the present invention there is provided a tip treatment bar for a gas turbine to which bar a coating is applied.
In an embodiment in accordance with the invention, the coating acts to dissipate strain energy generated by deflection of the bar as it vibrates. Consequently the amplitude of the vibrations is reduced, so combating high cycle fatigue failure.
The coating may be a hard ceramic coating, such as Magnesia Alumina Spinel. The coating may be applied directly to the material of the bar, for example by plasma spraying, but alternatively the coating may be applied to a substrate and which is subsequently bonded to the bar.
The tip treatment bars may be made individually and subsequently assembled with end supports to form a tip treatment ring. In another embodiment, a plurality of bars are formed as ring sections by injection moulding and these sections are assembled together to form the ring. Alternatively, the entire ring can be formed by appropriate machining of a single component.
In embodiments in which the hard coating is applied to a substrate, which is bonded onto the tip treatment bar, the substrate may be metal and the coating may be applied to the substrate by plasma spraying. This has the advantage that the coating can be applied to non metallic tip treatment bars, for example those made of composite material, such as an organic matrix composite material, which might not withstand the high temperatures of coating deposition processes such as plasma spraying. The coating may be applied, directly or indirectly, to at least one side of the tip treatment bar.